The present invention relates to a nitride-type, rare earth magnet material made of an Rxe2x80x94Txe2x80x94M(xe2x80x94B)xe2x80x94N alloy and an isotropic, bonded rare earth magnet formed from such a nitride-type, rare earth magnet material, particularly to a nitride-type, rare earth magnet material comprising Sm and La as R and an isotropic, bonded rare earth magnet having good magnetizability.
Bonded rare earth magnets comprising Ndxe2x80x94Fexe2x80x94B magnet powder have conventionally been used widely, though their applications at high temperatures are restricted because they have as low Curie temperatures as about 300xc2x0 C. and high temperature coefficients of coercivity iHc.
Sm2Fe17Nx compounds formed by making Sm2Fe17 compounds absorb nitrogen have recently been finding industrial applications as magnet powder for bonded magnets, because they show higher Curie temperatures (470xc2x0 C.) and anisotropic magnetic field (260 kOe) than those of Nd2Fe14B compounds. However, Sm2Fe17Nx compounds fail to show usefully high iHc unless they are pulverized to as small a particle size as a few xcexcm, corresponding to the size of a single magnetic domain. Also, Sm2Fe17Nx compounds in a state of fine powder having a few xcexcm size are easily oxidized in the air at room temperature, resulting in drastic deterioration of their magnetic properties. In addition, Sm2Fe17Nx compounds in a state of fine powder having a few xcexcm cannot be filled in the bonded magnets at high density, failing to achieve usefully high maximum energy products (BH)max.
To solve the above problems in connection with fine pulverization, Japanese Patent Laid-Open No. 4-260302 describes that nitrided magnet powder having a composition comprising 5-15 atomic % of Sm, 0-10 atomic % of M which is at least one element selected from the group consisting of Zr, Hf, Nb, Ta, W, Mo, Ti, V, Cr, Ga, Al, Sb, Pb and Si, and 0.5-25 atomic % of N, the balance being substantially Fe or Fe and Co (Fe content is 20 atomic % or more) is obtained by heat-treating the Sm2Fe17 compounds in a hydrogen atmosphere and then under reduced pressure and further nitriding it, and that when M is contained, the resultant magnet powder has an average crystal grain size of 1 xcexcm or less and an average particle size of 20 xcexcm or more, showing magnetic anisotropy. The inventors"" research has revealed, however, that the nitrided magnet powder produced under the conditions of Japanese Patent Laid-Open No. 4-260302 is magnetically isotropic, having an average crystal grain size of more than 1 xcexcm. The reason therefor is considered that a hydrogen absorption temperature in Examples of Japanese Patent laid-Open No. 4-260302 is as low as 650xc2x0 C., lower than a hydrogenation/decomposition temperature.
As a result of the inventors"" investigation, it has been found that nitrided magnet powder having an average particle size of 10 xcexcm or more and an average crystal grain size of 1 xcexcm or less can be produced, when thin ribbons obtained from a mother alloy melt for a nitride-type, rare earth magnet material by rapid quenching at as high a peripheral speed of a quenching roll as, for instance, 45 m/sec. or more are heat-treated under the conditions of Japanese Patent Laid-Open No. 4-260302 and then nitrided. However, because thin mother alloy ribbons rapidly-quenched under the above conditions are extremely as thin as less than 50 xcexcm, magnet powders obtained by finally nitriding them have ragged shapes, reflecting the shapes of the thin ribbons. As a result, such magnet powders cannot be compression-molded well. Accordingly, such nitrided magnet powder cannot be formed into isotropic, bonded magnets having as high a density as more than 6.1 g/cm3, making less likely the expectation that (BH)max is improved by increasing the filling density of the nitrided magnet powder.
Magnetizability is extremely important for isotropic, bonded rare earth magnets, and a magnetic field intensity for magnetization is preferably 25 kOe or less at room temperature in practical applications. However, conventional Rxe2x80x94Txe2x80x94Mxe2x80x94N-type, isotropic, bonded rare earth magnets are not well magnetized under the above conditions.
Accordingly, an object of the present invention is to provide a nitride-type, rare earth magnet material of an Rxe2x80x94Txe2x80x94M(xe2x80x94B)xe2x80x94N alloy, particularly a (Sm, La)xe2x80x94Txe2x80x94M(xe2x80x94B)xe2x80x94N alloy, wherein R is at least one rare earth element including Y, as a rare earth element Sm must be present, T is Fe alone or a combination of Fe and Co and/or Ni, and M is at least one element selected from the group consisting of Al, Ti, V, Cr, Mn, Cu, Ga, Zr, Nb, Mo, Hf, Ta, W and Zn, the nitride-type, rare earth magnet material containing an extremely small amount of xcex1-Fe, if any, and being substantially composed of a fine, hard magnetic phase of an R2T17-type structure.
Another object of the present invention is to provide an isotropic, bonded rare earth magnet containing such a nitride-type, rare earth magnet material and having good magnetizability.
With respect to nitride-type, rare earth magnet material powder and an isotropic, bonded rare earth magnet containing such magnet powder, research has been carried out to achieve the following objectives:
(1) To provide nitride-type, rare earth magnet material particles substantially composed of a hard magnetic phase of R2T17-type structure, xcex1-Fe being preferably 5% or less, more preferably 2% or less, particularly 0% by average area ratio;
(2) To provide a small decrease in magnetic properties by temperature elevation (good heat resistance);
(3) To provide high (BH)max;
(4) To easily form isotropic, bonded rare earth magnets under practical molding pressure;
(5) To provide bonded rare earth magnets with magnetizability sufficient for practical applications; and
(6) To provide bonded rare earth magnets having a density of more than 6.1 g/cm3.
As a result, it has been found that nitride-type, rare earth magnet materials satisfying the above requirements (1)-(6) can be produced by preparing by a melting method a mother alloy having a composition corresponding to the basic composition of an Rxe2x80x94Txe2x80x94M(xe2x80x94B)xe2x80x94N-type, nitrided rare earth magnet alloy, wherein R is at least one rare earth element including Y, Sm being indispensable, T is Fe alone or Fe and Co and/or Ni, and M is at least one element selected from the group consisting of Al, Ti, V, Cr, Mn, Cu, Ga, Zr, Nb, Mo, Hf, Ta, W and Zn, and subjecting the resultant mother alloy to a homogenizing heat treatment at 1010-1280xc2x0 C. for 1-40 hours in an inert gas atmosphere containing no nitrogen, if necessary, and then to a hydrogenation/decomposition reaction treatment, a dehydrogenation/recombination reaction treatment and a nitriding treatment in this order.
It has particularly been found that nitride-type, rare earth magnet materials satisfying the above requirements (1)-(6) can be produced by rapidly cooling a mother alloy melt having a composition corresponding to the basic composition of an Rxe2x80x94Txe2x80x94Mxe2x80x94Bxe2x80x94N nitride-type, magnet alloy, wherein R is at least one rare earth element including Y, as a rare earth element Sm must be present, T is Fe alone or a combination of Fe and Co and/or Ni, and M is at least one element selected from the group consisting of Al, Ti, V, Cr, Mn, Cu, Ga, Zr, Nb, Mo, Hf, Ta, W and Zn, wherein Ti must be present, at a peripheral speed of a quenching roll that is preferably 0.05-15 m/second, more preferably 0.08-10 m/second, particularly preferably 0.1-8 m/second, and then subjecting the resultant quenched alloy to a hydrogenation/decomposition reaction treatment and a dehydrogenation/recombination reaction treatment described below, and then to a nitriding treatment. It has further been found that a combination of Sm and La is advantageously selected as the R element to improve the magnetizability. The present invention has been completed based on these findings.
Thus, the nitride-type, rare earth magnet material according to the present invention has a basic composition represented by:
R60 T100-(xcex1+xcex2+xcex3+xcex4)Mxcex2Bxcex3Nxcex4 (atomic %),
wherein R is at least one rare earth element including Y, as a rare earth element Sm must be present, T is Fe alone or a combination of Fe and Co and/or Ni, M is at least one element selected from the group consisting of Al, Ti, V, Cr, Mn, Cu, Ga, Zr, Nb, Mo, Hf, Ta, W and Zn, 6xe2x89xa6xcex1xe2x89xa615, 0.5xe2x89xa6xcex2xe2x89xa610, 0xe2x89xa6xcex34, and 4xe2x89xa6xcex4xe2x89xa630, the nitride-type, rare earth magnet material being substantially composed of a hard magnetic phase of an R2T17-type structure having an average crystal grain size of 0.01-1 xcexcm, and an average area ratio of xcex1-Fe being 5% or less.
The nitride-type, rare earth magnet material according to a preferred embodiment of the present invention has a basic composition in which M is at least one element selected from the group consisting of Al, Ti, V, Cr, Mn, Cu, Ga, Zr, Nb, Mo, Hf, Ta, W and Zn, wherein Ti must be present, and 6xe2x89xa6xcex1xe2x89xa615, 0.5xe2x89xa6xcex2xe2x89xa610, 0xe2x89xa6xcex34, and 4xe2x89xa6xcex4xe2x89xa630. This basic composition provides a mother alloy with an average xcex1-Fe area ratio of 5% or less without a homogenizing heat treatment. In this case, the content (xcex2) of the M element including Ti should be 0.5-10 atomic %, more preferably 1-6 atomic %, particularly preferably 1-4 atomic %, and that the content of Ti should be 0.5 atomic % or more.
The nitride-type, rare earth magnet material according to another preferred embodiment of the present invention has a basic composition represented by (Sm, La)xcex1T100-(xcex1+xcex2+xcex3+xcex4)Mxcex2Bxcex3Nxcex4 (atomic %), wherein T is Fe alone or a combination of Fe and Co and/or Ni, M is at least one element selected from the group consisting of Al, Ti, V, Cr, Mn, Cu, Ga, Zr, Nb, Mo, Hf, Ta, W and Zn, and 6xe2x89xa6xcex1xe2x89xa615, 0.5xe2x89xa6xcex2xe2x89xa610, 0xe2x89xa6xcex34, and 4xe2x89xa6xcex4xe2x89xa630. This nitride-type, rare earth magnet material substantially has a hard magnetic phase of a (Sm, La)2T17-type structure having a average crystal grain size of 0.01-1 xcexcm, and an average area ratio of xcex1-Fe being 5% or less. The content of La is preferably 0.05-1 atomic % per 100 atomic % of the overall basic composition.
In a further preferred embodiment of the present invention, the hard magnetic phase is composed of a mixed crystal of a rhombohedral crystal having a Th2Zn17-type structure and a hexagonal crystal having a Th2Ni17-type structure.
In a still further preferred embodiment of the present invention, the nitride-type, rare earth magnet material is in the form of powder having a one-peak particle size distribution with an average particle size of 10-300 xcexcm.
In a still further preferred embodiment of the present invention, the nitride-type, rare earth magnet material contains as inevitable impurities 0.25 weight % or less of oxygen and 0.1 weight % or less of carbon.
The bonded rare earth magnet according to the present invention is produced by bonding the above nitride-type, rare earth magnet material powder with a binder resin. The binder resin is preferably a thermosetting resin. The bonded rare earth magnet preferably has a density of more than 6.1 g/cm3 by compression molding and a subsequent heat curing treatment.